U.S. patent number 8,619,319 [Application Number 12/896,523] was granted by the patent office on 2013-12-31 for image processor and image processing method.
This patent grant is currently assigned to Canon Kabushiki Kaisha. The grantee listed for this patent is Takashi Fujita, Fumihiro Goto, Fumitaka Goto, Tomokazu Ishikawa, Rie Kajihara, Akihiko Nakatani, Mitsuhiro Ono, Ayumi Sano, Okinori Tsuchiya, Akitoshi Yamada. Invention is credited to Takashi Fujita, Fumihiro Goto, Fumitaka Goto, Tomokazu Ishikawa, Rie Kajihara, Akihiko Nakatani, Mitsuhiro Ono, Ayumi Sano, Okinori Tsuchiya, Akitoshi Yamada.
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United States Patent |
8,619,319 |
Tsuchiya , et al. |
December 31, 2013 |
Image processor and image processing method
Abstract
Inputted image data is converted to M number of multi-value data
having a lower resolution than the inputted image data, and after
quantization processing has been performed for each of the M number
of multi-value data, an image is printed by M number of relative
movements (M-pass printing) that corresponds to the M number of
quantized data. By doing so, when compared with the case in which a
resolution reduction process is not performed, it is possible to
suppress the number of pixels that become the object of
quantization processing, and it becomes possible to output an image
with no fluctuation in image density or density unevenness without
a decrease in the processing speed.
Inventors: |
Tsuchiya; Okinori (Yokohama,
JP), Goto; Fumihiro (Kawasaki, JP), Yamada;
Akitoshi (Yokohama, JP), Nakatani; Akihiko
(Kawasaki, JP), Ono; Mitsuhiro (Tokyo, JP),
Goto; Fumitaka (Tokyo, JP), Fujita; Takashi
(Kawasaki, JP), Kajihara; Rie (Minoo, JP),
Sano; Ayumi (Kawasaki, JP), Ishikawa; Tomokazu
(Kawasaki, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Tsuchiya; Okinori
Goto; Fumihiro
Yamada; Akitoshi
Nakatani; Akihiko
Ono; Mitsuhiro
Goto; Fumitaka
Fujita; Takashi
Kajihara; Rie
Sano; Ayumi
Ishikawa; Tomokazu |
Yokohama
Kawasaki
Yokohama
Kawasaki
Tokyo
Tokyo
Kawasaki
Minoo
Kawasaki
Kawasaki |
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
43854612 |
Appl.
No.: |
12/896,523 |
Filed: |
October 1, 2010 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20110085183 A1 |
Apr 14, 2011 |
|
Foreign Application Priority Data
|
|
|
|
|
Oct 9, 2009 [JP] |
|
|
2009-235328 |
|
Current U.S.
Class: |
358/1.2;
358/3.02 |
Current CPC
Class: |
H04N
1/40068 (20130101) |
Current International
Class: |
G06F
15/00 (20060101); G06K 1/00 (20060101); G06K
15/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2000-103088 |
|
Apr 2000 |
|
JP |
|
2003-53963 |
|
Feb 2003 |
|
JP |
|
2008-162151 |
|
Jul 2008 |
|
JP |
|
2009-148988 |
|
Jul 2009 |
|
JP |
|
Other References
US. Appl. No. 12/952,819, filed Nov. 23, 2010. cited by applicant
.
U.S. Appl. No. 12/962,044, filed Dec. 7, 2010. cited by applicant
.
U.S. Appl. No. 12/896,539, filed Oct. 1, 2010. cited by applicant
.
U.S. Appl. No. 12/942,718, filed Nov. 9, 2010. cited by applicant
.
Japanese Office Action dated Jan. 29, 2013 issued during
prosecution of related Japanese application No. 2009-235328. cited
by applicant.
|
Primary Examiner: Tran; Douglas
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. An image processor for processing input image data that
corresponds to a pixel area in order to perform printing in the
pixel area on a printing medium by a plurality of relative
movements of a printing unit for printing a same color dot, with
respect to a printing medium, comprising: a first generation unit
configured to generate a plurality of multi-value data for the same
color corresponding to the plurality of relative movements, that
have a lower resolution than the input image data, based on the
input image data; and a second generation unit configured to
generate a plurality of binary data for the same color having a
higher resolution than the multi-value data corresponding to the
plurality of relative movements, based on the plurality of
multi-value data.
2. The image processor according to claim 1, wherein the resolution
of the binary data is equal to the resolution of the input image
data.
3. The image processor according to claim 1, wherein the first
generation unit generates the multi-value image data having the
lower resolution than the input image data by performing an
averaging process of a plurality of the input image data
corresponding to a plurality of pixel areas.
4. The image processor according to claim 1, wherein the first
generation unit generates the multi-value image data having the
lower resolution than the input image data by performing an
averaging process of a plurality of the multi-value image data for
the same color corresponding to a plurality of pixel areas.
5. The image processor according to claim 1, wherein the second
generation unit generates a plurality of quantized data
corresponding to the plurality of relative movements by performing
quantization processing to each of the plurality of multi-value
data, wherein each of the plurality of the quantized data has a
lower number of levels for representing a gradation of the pixel
area than that of the multi-value data; and generates the plurality
of the binary data, by performing binarization processing to each
of the plurality of the quantized data corresponding to the
plurality of relative movements.
6. The image processor according to claim 5, wherein the second
generation unit generates the binary data having the higher
resolution than the quantized data by determining printing dot or
not printing dot to a plurality of sub areas in the pixel area
respectively according to the number of levels of the quantized
data corresponding to the pixel area.
7. The image processor according to claim 1, wherein the second
generation unit generates the binary data having the higher
resolution than the multi-value data by determining printing dot or
not printing dot to a plurality of sub areas in the pixel area
respectively according to a value represented by the multi-value
data corresponding to the pixel area.
8. The image processor according to claim 7, wherein the second
generation unit generates the binary data by determining printing
dot or not printing dot to a plurality of sub areas respectively,
by referencing a dot arrangement pattern determined for a number of
dots and arrangement of dots in the pixel area.
9. An image processor for processing input image data that
corresponds to a pixel area in order to perform printing in the
pixel area on a printing medium by a plurality of relative
movements of a printing unit for printing a same color dot, with
respect to the printing medium, comprising: a setting unit
configured to set one mode from among a plurality of printing modes
that include a first mode for printing in the pixel area by
performing M (where M is an integer larger than 2) number of
relative movements, and a second mode for printing in the pixel
area by performing N (where N is an integer larger than 3 and
N>M) number of relative movements; and a processing unit
configured to, when the first mode has been set by the setting
unit, (a) generate a plurality of multi-value image data for the
same color corresponding to the plurality of relative movements,
that have a lower resolution than the input image data, based on
the input image data, (b) generate a plurality of quantized data
corresponding to the plurality of relative movements, by performing
quantization processing to each of the plurality of multi-value
image data for the same color corresponding to the plurality of
relative movements, and (c) generate a plurality of binary data
corresponding to the plurality of relative movements, that have a
higher resolution than the quantized data by performing
binarization processing to each of the plurality of quantized data
corresponding to the plurality of relative movements, and, when the
second mode has been set by the setting unit, (d) generate a binary
data for the same color by performing binarization processing of
the input image data, and (e) divide the binary data for the same
color to divisions corresponding to the plurality of relative
movements.
10. An image processor for processing input image data that
corresponds to a pixel area in order to perform printing in the
pixel area on a printing medium by a plurality of relative
movements of a printing unit for printing a same color dot, with
respect to the printing medium, comprising: a setting unit
configured to set one mode from among a plurality of printing modes
that include a first mode for printing in the pixel area by
performing M (where M is an integer larger than 2) number of
relative movements, and a second mode for printing in the pixel
area by performing N (where N is an integer larger than 3 and
N>M) number of relative movements; and a processing unit
configured to, when the first mode has been set by the setting
unit, (a) generate a plurality of multi-value image data for the
same color corresponding to the plurality of relative movements,
that have a lower resolution than the input image data, based on
the input image data; and (b) generate a plurality of binary data
that have a higher resolution than the multi-value data
corresponding to the plurality of relative movements, based on the
plurality of multi-value data for the same color corresponding to
the plurality of relative movements, and, when the second mode has
been set by the setting unit, (c) generate a plurality of
multi-value data corresponding to the plurality of relative
movements, that have a resolution equal to the resolution of the
input image data, based on the input image data, and (d) generate a
plurality of binary data corresponding to the plurality of relative
movements, by performing binarization processing to each of the
plurality of multi-value data for the same color corresponding to
the plurality of relative movements.
11. The image processor according to claim 10, wherein the
processing unit is further configured to, when the first mode has
been set by the setting unit, (e) generate a plurality of quantized
data corresponding to the plurality of relative movements, by
performing quantization processing to each of the plurality of
multi-value data for the same color corresponding to the plurality
of relative movements, and (f) generate the plurality of binary
data corresponding to the plurality of relative movements by
performing binarization processing to each of the plurality of
quantized data corresponding to the plurality of relative
movements.
12. An image processing method for processing input image data that
corresponds to a pixel area in order to perform printing in the
pixel area on a printing medium by a plurality of relative
movements of a printing unit for printing a same color dot, with
respect to the printing medium, comprising: a step of generating a
plurality of multi-value image data for the same color
corresponding to the plurality of relative movements, that have a
lower resolution than the input image data, based on the input
image data; and a step of generating a plurality of binary data
that have a higher resolution than the multi-value data for the
same color corresponding to the plurality of relative movements,
based on the plurality of multi-value data.
13. An image processor for processing input image data that
corresponds to a pixel area in order to perform printing in the
pixel area on a printing medium by a relative movement of a
plurality of nozzle arrays in which nozzles for printing same color
dot are arrayed, with respect to a printing medium, comprising: a
first generation unit configured to generate a plurality of
multi-value data for the same color corresponding to the plurality
of nozzle arrays, that have a lower resolution than the input image
data, based on the input image data; and a second generation unit
configured to generate a plurality of binary data for the same
color having a higher resolution than the multi-valued data for the
same color corresponding to the plurality of nozzle arrays, based
on the plurality of multi-valued data.
14. The image processor according to claim 13, wherein the second
generation unit generates the plurality of quantized data
corresponding to the plurality of nozzle arrays by performing
quantization processing to each of the plurality of multi-value
data, wherein each of the plurality of the quantized data has a
lower number of levels for representing a gradation of the pixel
area than that of the multi-value data, and generates the plurality
of the binary data, by performing binarization processing to each
of the plurality of the quantized data corresponding to the
plurality of nozzle arrays.
15. The image processor according to claim 14, wherein the second
generation unit generates the binary data having a higher
resolution than the quantized data by determining printing dot or
not printing dot to a plurality of sub areas in the pixel area
respectively according to the number of levels of the quantized
data corresponding to the pixel area.
16. The image processor according to claim 15, wherein the second
generation unit generates the binary data by determining printing
dot or not printing dot to a plurality of sub areas respectively by
referencing a dot arrangement pattern determined for a number of
dots and arrangement of dots in the pixel area.
17. The image processor according to claim 13, wherein the
resolution of the binary data is equal to the resolution of the
input image data.
18. The image processor according to claim 13, wherein the first
generation unit generates the multi-value image data having the
lower resolution than the input image data by performing an
averaging process of a plurality of the input image data
corresponding to a plurality of pixel areas.
19. The image processor according to claim 13, wherein the first
generation unit generates the multi-value image data having the
lower resolution than the input image data by performing an
averaging process of a plurality of the multi-value image data for
the same color corresponding to a plurality of pixel areas.
20. The image processor according to claim 13, wherein the second
generation unit generates the binary data having a higher
resolution than the multi-valued data by determining printing dot
or not printing dot to a plurality of sub areas in the pixel area
respectively according to a value represented by the multi-value
data corresponding to the pixel area.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processor and image
processing method that process multi-value image data that
correspond to the same area in order to print an image in the same
area by relatively moving a printing unit or a plurality of
printing element groups a plurality of times with respect to the
same area of a printing medium.
2. Description of the Related Art
In inkjet printing devices, a multipass printing method that prints
by performing a plurality of printing scans of image data to be
printed in the same area of a printing medium is known as a
technique for reducing density unevenness and stripes. However,
recently, by performing multipass printing, a new adverse effect of
image due to deviation of print position among printing scans is
viewed with suspicion. Here, deviation of the printing position
among printing scans is, for example, deviation between a dot group
that is printed in a first printing scan and a dot group that is
printed in the second printing scan in the same area as the first
printing scan. Deviation of these dot groups is caused by
fluctuation in the distance between the printing medium and
ejection face (paper distance), fluctuation in the conveyed amount
of the printing medium, and the like, when this happens, the dot
coverage rate on the paper fluctuates, which causes density
fluctuation and density unevenness of an image. Therefore,
recently, a method for processing image data during multipass
printing that is capable of handling deviation in the printing
position between planes that occurs due to these kinds of
fluctuations in printing conditions is desired.
In Japanese Patent Laid-Open No. 2000-103088, an image data
processing method is disclosed for suppressing adverse effects to
an image as described above. This patent application focuses on the
cause of fluctuation in image density caused by fluctuation in
various printing conditions being that the binary image data that
are printed in different printing scans are in a complementary
relationship with each other. In addition, in order to solve this
problem, this patent application discloses a method for dividing
image data in the multi-value image data stage before binarization
in order to correspond to different printing scans, and then
independently (uncorrelated) binarizing the multi-value image data
after being divided.
FIG. 8A is a diagram that illustrates the configuration of dots
when image data is divided and binarized according to the method of
Japanese Patent Laid-Open No. 2000-103088. In this figure, the
black dots 1501 are printed in the first printing scan, the white
dots 1502 are printed in the second printing scan, and the gray
dots 1503 are printed the first printing scan and second printing
scan overlap. Here, there is no complementary relationship between
the dots that are printed in the first printing scan and the dots
that are printed in the second printing scan. Therefore, there are
portions (gray dots) 1503 where two dots overlap, and there are
also blank areas where no dots are printed.
In this kind of dot arrangement, the dot coverage rate on the
printing medium does not fluctuate much even though the dot group
that is printed in the first printing scan and the dot group that
is printed in the second printing scan shift the amount of one
pixel in the main scanning direction or sub scanning direction.
This is because new areas where dots printed in the first printing
scan overlap dots printed in the second printing scan appear,
however; there are also portions where two dots that were
originally supposed to overlap no longer overlap. Therefore, when
performing judgment over a somewhat large area, the dot coverage on
the printing medium does not fluctuate much, and thus it is also
difficult for changes to occur in the image density. In other
words, by employing the method of Japanese Patent Laid-Open No.
2000-103088, it is possible to suppress the occurrence of density
fluctuation and density unevenness in an image even when
fluctuation in the distance between the printing medium and
ejection face (paper distance), or fluctuation in the conveying
amount of the printing medium occurs.
However; when using the method disclosed in Japanese Patent
Laid-Open No. 2000-103088, the number of pixels that are to be the
object of quantization processing increases by the amount of the
divided planes. Therefore, it takes more time to complete
quantization when compared with the case in which quantization is
performed for just one gradation plane, and thus a new problem
occurs in that the processing speed becomes slower. In order to
avoid this problem, a large capacity memory can be prepared, or a
CPU can be prepared that has superior processing capability,
however; the cost of the device then increases.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the problems
described above. In other words, the object of the present
invention is to provide an image processor and image processing
method that are capable of outputting images in multipass printing
that have no fluctuation in image density or density unevenness due
to deviation during printing, and are capable of outputting such
images without a drop in processing speed or increase in cost.
The first aspect of the present invention is an image processor
that processes input image data that corresponds to a pixel area in
order to print dots in the pixel area on a printing medium by M
number of relative movements between a printing unit that prints
dots and the printing medium, comprising: a unit configured to
convert the input image data to M number of multi-value data,
having lower resolution than the input image data, in order to
correspond to the M number of relative movements; a unit configured
to quantize each of the M number of multi-value data and to create
M number of quantized data having a lower number of levels than the
multi-value data; and a unit configured to binarize each of the M
number of quantized data, and create M number of binary data having
a higher resolution than the quantized data so that the printing
unit can print dots in the pixel area by the M number of relative
movements.
The second aspect of the present invention is an image processor
that processes input image data that corresponds to a pixel area in
order to print in the pixel area on a printing medium by M number
of relative movements between a printing unit that prints dots and
the printing medium, comprising: a setting unit configured to set
one mode from among a plurality of printing modes that include a
first mode for printing in the pixel area by performing M number of
relative movements, and a second mode for printing in the pixel
area by performing N (N>M) number of relative movements; a unit
configured to convert the input image data to M number of
multi-value data, having a resolution that is lower than the
resolution of the input image data, in order to correspond to the M
number of relative movements when the first mode has been set by
the setting unit; a unit configured to quantize each of the M
number of multi-value data and create M number of first quantized
data having a number of levels that is less than the multi-value
data; a unit configured to binarize each of the M number of first
quantized data, and create M number of binary data having a higher
resolution than the first quantized data so that the printing unit
can print dots in the pixel area by the M number of relative
movements; a unit configured to quantize the input mage data and
create second quantized data when the second mode has been set by
the setting unit; and a unit configured to divide the second
quantized data into N number of divisions and create N number of
binary data so that the printing unit can print dots in the pixel
area by N number of relative movements.
The third aspect of the present invention is An image processing
method that processes input image data that corresponds to a pixel
area in order to print dots in that pixel area on a printing medium
by M number of relative movements between a printing unit that
prints dots and the printing medium, comprising the steps of:
converting the input image data to M number of multi-value data,
having lower resolution than the input image data, in order to
correspond to the M number of relative movements; quantizing each
of the M number of multi-value data, and creating M number of
quantized data having a lower number of levels than the multi-value
data; and binarizing each of the M number of quantized data, and
creating M number of binary data having a higher resolution than
the quantized data so that the printing unit can print dots in the
pixel area by the M number of relative movements.
Further features of the present invention will become apparent from
the following description of exemplary embodiments (with reference
to the attached drawings).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram for explaining the steps of image
processing of a first embodiment when completing an image in the
same area of a printing medium by 2-pass multipass printing;
FIGS. 2A and 2B are perspective diagrams and illustrate a photo
direct printer of an embodiment of the present invention, and the
printing unit of a printer engine;
FIG. 3 is a block diagram that illustrates the configuration of PD
printer control according to an embodiment of the present
invention;
FIGS. 4A to 4H are diagrams for explaining the dot overlap
rate;
FIG. 5 is a diagram that illustrates one example of a mask that can
be used in the present invention;
FIG. 6 is a diagram that illustrates the relationship between the
distribution rate and dot overlap rate;
FIGS. 7A and 7B are diagrams that illustrate one example of an
error-diffusion matrix that is used in quantization processing;
FIGS. 8A to 8C are diagrams for explaining the relationship between
the dot overlap state or the dot dispersion state and
graininess;
FIG. 9 is a flowchart for explaining the processing steps that are
executed by the quantization processing unit of a third
embodiment;
FIG. 10 is a diagram for explaining the dot overlap rate when index
expansion processing is performed;
FIGS. 11A and 11B are diagrams for explaining the discharge outlet
array state of a printing head that can be applied to embodiments
of the invention;
FIG. 12 is a schematic diagram for explaining the dot arrangement
pattern and data conversion method;
FIG. 13 is a diagram for explaining the correlation between one
example of results from a quantization process, which is performed
using threshold values that are entered in a specified threshold
value table, and input values (K1ttl and K2ttl);
FIG. 14 is a block diagram showing the relationship of FIGS. 14A
and 14B;
FIG. 14A and FIG. 14B are block diagrams for explaining the steps
of image processing that is executed in a second embodiment;
FIG. 15 is a block diagram showing the relationship of FIGS. 15A
and 15B;
FIG. 15A and FIG. 15B are block diagrams for explaining the steps
of image processing executed in a third embodiment;
FIG. 16 is a diagram that illustrates one example of a dot
arrangement pattern that is used in an index expansion process;
FIG. 17 is a diagram that illustrates one example of a dot
arrangement pattern that is used in an index expansion process;
and
FIG. 18 is a diagram of a detailed example of the image processing
illustrated in FIG. 1.
DESCRIPTION OF THE EMBODIMENTS
In the following, embodiments of the present invention will be
explained in detail with reference to the accompanying
drawings.
The embodiments explained below use an inkjet printer as an
example, however; the present invention is not limited to an inkjet
printer. The invention can be applied to devices other than an
inkjet printer as long as the device can print an image on a
printing medium via a printing unit that prints dots while moving
relative to the printing medium.
The "relative movement (or relative scanning) between the printing
unit and the printing medium is an operation of relatively moving
(scanning) the printing unit with respect to the printing medium,
or an operation of relatively moving (conveying) the printing
medium with respect to the printing unit. The printing unit is one
or more printing element group (nozzle array) or one or more
printing head.
In the image processor explained below, data processing is
performed in order to print images in the same area by relatively
moving a printing unit a plurality of times, or relatively moving a
plurality of printing element groups with respect to the same area
of the printing medium (specified area). Here, the "same area
(specified area)" is "one pixel area" on a micro scale, or on a
macro scale, "an area that can be printed by one relative
movement". That the area "may also be called a pixel area (simply a
"pixel")" means that the area is the minimum unit for which
gradation expression is possible using multi-value image data. On
the other hand, "an area that can be printed by one relative
movement" is an area on the printing medium that the printing unit
passes over during one relative movement, or is an area smaller
than that area (for example, one raster area). For example, when
executing the multipass mode as illustrated in FIG. 10, on a macro
scale, one print area in the figure can also be defined as the same
area.
<Summary Explanation of the Printer>
FIG. 2A is a perspective drawing of a photo direct printer
(hereafter referred to as a PD printer) 1000 of an embodiment of
the present invention, or in other words, an image formation device
(image processor). The PD printer 1000 comprises: a normal function
of receiving and printing data from a host computer (PC), a
function of directly reading and printing an image that is stored
in a memory medium such as a memory card, and a function of
receiving and printing an image from a digital camera, PDA or the
like.
In FIG. 2A, 1004 is a discharge tray into which printed paper can
be stacked, and 1003 is an access cover that a user can open or
close when replacing the printing head cartridge or ink tank that
are stored in the main unit. Menu items for setting various
conditions related to printing (for example, the type of printing
medium, image quality, and the like) are displayed on the control
panel 1010 that is provided on the upper case 1002, and a user can
set these items according to the type and use of the image to be
output.
In the figure, 1007 is an auto supply unit that automatically
supplies printing medium to the main unit, 1009 is a card slot into
which an adapter to which a memory can be mounted is inserted, and
1012 is a USB terminal for connecting a digital camera. A USB
connector for connecting a PC is provided on the rear surface of
the PD printer 1000.
<Electrical Specifications of the Control Unit>
FIG. 3 is a block diagram that illustrates the construction of the
main parts for control of the PD printer 1000 of embodiments of the
present invention. In FIG. 3, the same reference numbers are used
for parts that are common with those in drawings described above,
and explanations of those parts area omitted. As will be made clear
in the explanation below, the PD printer 1000 functions as an image
processor.
In FIG. 3, 3000 is a control unit (control board) and 3001 is an
image processing ASIC (special customized LSI). In the figure, 3002
is a digital signal processor (DSP) having an internal CPU, and
controls the various control processing and various image
processing described later. Reference number 3003 is a memory
having a program memory 3003a that stores the control programs for
the CPU of the DSP 3002, a RAM area that stores programs during
execution, and a memory area that functions as a work memory and
stores image data. Reference number 3004 is a printer engine, and
here a printer engine is mounted for an inkjet printer that prints
color images using a plurality of colors of ink. In the figure,
3005 is a USB connector used as a port for connecting a digital
camera (DSC) 3012. Reference number 3006 is a connector for
connecting a viewer 1011. Number 3008 is a USB hub, and when the PD
printer 1000 prints based on image data from a PC 3010, lets the
data pass as is from the PC 3010 to the printer engine 3004 via the
USB 3021. In doing so, the connected PC 3010 can execute printing
by exchanging data and signals directly with the printer engine
3004 (functions as a typical PC printer).
Reference number 3009 is a power-supply connector, and DC voltage
that is converted from the commercially available AC is input from
the power supply 3019. The PC 3010 is a typical personal computer,
3011 is a memory card (PC card) as mentioned above, and 3012 is a
digital camera (DSC). The exchange of signals between this control
unit 3000 and the printer engine 3004 is performed via the USB 3021
described above, or an IEEE-1284 bus 3022.
<Summary of the Printing Unit>
FIG. 2B is a perspective drawing that illustrates the printing unit
of the printer engine unit of a serial-type inkjet printer of the
embodiments of the present invention. The auto feed unit 1007 feeds
printing medium P to the nip section of a conveying roller 5001 and
pinch roller 5002 that follows it, which are located on the
conveying path. After that, the printing medium P is conveyed by
the rotation of the conveying roller 5001 in the direction of arrow
A in the figure (sub scanning direction) while being supported and
guided over the platen 5003. The pinch roller 5002 is elastically
pressed against the conveying roller 5001 by a pressure unit such
as a spring (not shown in the figure). The conveying roller 5001
and pinch roller 5002 are components of a first conveying unit on
the up-line side in the conveying direction of the printing
medium.
The platen 5003 is located at the printing position that faces a
surface (ejection face) on which the discharge outlets of the
inkjet type printing head 5004, and by supporting the rear surface
of the printing medium P, the distance between the rear surface of
the printing medium P and the ejection face is kept at a constant
distance. The printing medium P that is conveyed over the platen
5003 and on which printing is performed is held between a rotating
discharge roller 5005 and spurs 5006, which are rotating bodies
that follow the discharge roller 5005, conveyed in the A direction
and discharged to the paper discharge tray 1009 from the platen
5003. The discharge roller 5005 and spurs 5006 are components of a
second conveying unit that is located down-line in the conveying
direction of the printing medium.
The printing head 5009, with the ejection face facing the platen
5003 and printing medium P, is mounted in a carriage 5008 such that
it is removable. The carriage 5008 is driven back and forth by the
driving force of a carriage motor E0001 along two guide rails 5009
and 5010, and while moving, the printing head 5004 executes the ink
discharge operation according to a print signal. The direction that
the carriage 5008 moves in is the direction that crosses the
direction (direction of arrow A) in which the printing medium is
conveyed, and is called the main scanning direction. On the other
hand, the conveying direction of the printing medium is called the
sub scanning direction. Printing is performed on the printing
medium P by alternately repeating the main scan by the carriage
5008 and printing head 5004 (movement that accompanies printing)
and conveying of the printing medium (sub scan).
FIG. 11A is diagrams illustrating the case in which the printing
head 5004 used in the embodiments is observed from the discharge
outlet formation surface. In the figure, 41 is a Cyan nozzle array
(print element group), 92 is a Magenta nozzle array, 43 is a Yellow
nozzle array and 44 is a Black nozzle array. The width in the sub
scanning direction of each nozzle array is d, and printing having a
width d is possible in one scan.
Each of the nozzle arrays 41 to 44 is such that a plurality of
nozzles that discharge ink drops is arranged in the sub scanning
direction at intervals of 600 dpi (dots/inch), or in other words,
approximately 42 .mu.m. Each individual nozzle comprises a
discharge outlet that is the outlet when discharging ink, an ink
path for directing the ink to the discharge outlet, and
electrothermal conversion element that causes film boiling to occur
in the ink near the discharge outlet. With this kind of
construction, by applying a voltage pulse to the individual
electrothermal conversion elements according to a discharge signal,
film boiling occurs in the ink near the electrothermal conversion
elements, and an amount of ink corresponding to the growth of
generated bubbles becomes a suitable amount of liquid droplets and
is discharged from each of the discharge outlets.
The printing apparatus of this embodiment is able to execute
multipass printing, so that in the area that can be printed during
one printing scan by the printing head 5004, an image is gradually
formed by performing printing scanning a plurality of times. When
doing this, by performing the conveying operation an amount smaller
than the width of the printing head 5004 between each printing
scan, density unevenness or stripes due to variation in each
individual nozzle can be reduced.
Embodiment 1
FIG. 1 is a block diagram for explaining the steps of the image
processing of an embodiment in which an image is completed in the
same area of a printing medium by using 2-pass multipass printing.
In this embodiment, the control unit 3000 comprises a multi-value
image data input unit (61), color conversion/image data division
unit (62), processing resolution setting unit (60), gradation
correction processing units (63-1, 63-2), and quantization
processing units (65-1, 65-2).
In this embodiment, the RGB multi-value image data that is input to
the multi-value image data input unit 61 from an external device is
taken to be image data in which individual pixel for each color are
expressed in 8 bits (256 values) and have a resolution of 600
dpi.times.600 dpi. In addition, the color conversion/image data
division unit 62 converts this input image data (256-value RGB
data) to 2 groups of multi-value image data (256-value density
data) that corresponds to the ink color (CMYK). In this embodiment,
construction is such that the resolution of the density data from
the color conversion/image data division unit 62 is adjusted
according to the resolution set by the processing resolution
setting unit 60. For example, when the processing resolution
setting unit 60 designates a resolution of 600 dpi.times.300 dpi,
the color conversion/image data division unit 62 outputs two groups
of 256-value density data corresponding to the ink color (CMYK) at
a resolution of 600 dpi.times.300 dpi.
Continuing, in the gradation correction processing units (63-1,
63-2) and quantization processing units (65-1, 65-2), image process
is performed at 600 dpi.times.300 dpi. Therefore, in each
processing unit, the number of pixels to be processed is reduced by
half compared with when performing processing at 600 dpi.times.600
dpi. However, due to the division process, the object of processing
is doubled to two planes, so as a result, the number of times that
processing is performed is the same as the conventional method in
which the division process was not performed (method of Japanese
Patent Laid-Open No. 2000-103088) is not employed), so it is
possible to avoid the new problem that was explained in the section
of background of the invention. In FIG. 1, an example is given in
which the processing resolution setting unit 60 designates a
resolution of 600 dpi.times.300 dpi, however; the resolution of the
multi-value density data that is output from the color
conversion/image data division unit 62 can be set case by case.
This kind of resolution conversion process by the color
conversion/image data division unit 62 is possible by performing an
averaging process according to the resolution designated by the
processing resolution setting unit 60. For example, when the
designated resolution is 600 dpi.times.300 dpi as in this example,
the color conversion/image data division unit 62 first calculates
the average value of input image data (RGB) of two adjacent pixels
in the vertical direction (sub scanning direction) among the pixels
arranged 600 dpi.times.600 dpi. Then by referencing a 3-dimensional
lookup table (LUT) that is prepared in advance, it is possible to
obtain multi-value data (CMYK) for the first printing scan, and
multi-value data (CMYK) for the second printing scan.
In a different method, it is possible to reference a 3-dimensional
LUT for a 600 dpi.times.600 dpi as is, and to first obtain
multi-value data (CMYK) for the first printing scan, and
multi-value data (CMYK) for the second printing scan having a 600
dpi.times.600 dpi resolution. In this case, by calculating the
average value of two adjacent pixels in the vertical direction of
the 600 dpi.times.600 dpi CMYK values obtained in that way, it is
possible to obtain the 600 dpi.times.300 dpi multi-value data for
the first printing scan and for the second printing scan. In either
case, it is possible to use the same 3-dimensional LUT regardless
of the resolution that was designated by the processing resolution
setting unit 60.
The processing after the gradation correction processing is
performed independently and simultaneously for each color CMYK, so
the following explanation will be for just one gradation color (for
example, black). The 600 dpi.times.300 dpi multi-value data that
was generated by the color conversion/image data division unit 62
is sent to the respective gradation correction processing unit 63-1
and 63-2, where gradation correction processing is performed using
a 1-dimensional LUT. As a result, 600 dpi.times.300 dpi multi-value
data 64-1 for the first scan is output from the gradation
correction processing unit 63-1, and 600 dpi.times.300 dpi
multi-value data 64-2 for the second scan is output from the
gradation correction processing unit 63-2.
The multi-value data 64-1 for the first scan and the multi-value
data 64-2 for the second scan further undergo quantization
processing by the quantization processing units 65-1 and 65-2. In
the quantization processing unit 65-1 of this embodiment, the 600
dpi.times.300 dpi multi-value data 64-1 for the first scan that is
represented by 8-bit 256 values is converted to 600 dpi.times.300
dpi quantized (3-value) data 66-1 for the first scan that is
represented by 2-bit 3 values. In the quantization processing unit
65-2 of this embodiment, the 600 dpi.times.300 dpi multi-value data
64-2 for the second scan that is represented by 8-bit 256 values is
converted to 600 dpi.times.300 dpi quantized (3-value) data 66-2
for the second scan that is represented by 2-bit 3 values. The
quantization method that is employed by these two quantization
processing units 65-1 and 65-2 is a typical multi-value (3-value)
error diffusion method.
When doing this, a certain number of locations of overlapping dots
that are printed during two scans occurs, so in two error diffusion
processes, it is preferred that different diffusion matrices be
used. For example, in the quantization processing unit 65-1, the
diffusion matrix illustrated in FIG. 7A can be used, and in the
quantization processing unit 66-2, the diffusion matrix illustrated
in FIG. 7B can be used.
After 3-value image data 66-1 and 66-2 are obtained from the
quantization processing units 65-1 and 65-2, the data are sent to
the printer engine 3004 illustrated in FIG. 3 via an IEEE 1284 bus
3022. The following processing is executed by the printer engine
3004.
In the printer engine 3004, the 3-value data 66-1 for the first
scan undergoes processing by the first scan index expansion
processing unit 67-1 and is converted to 600 dpi.times.600 dpi
2-value data 68-1 for the first scan that is expressed by 1-bit 2
values. In addition, the 3-value data 66-2 for the second scan
undergoes processing by the second scan dot arrangement pattern
processing unit 67-2 and is converted to 600 dpi.times.600 dpi
2-value data 68-2 for the second scan that is expressed by 1-bit 2
values.
FIG. 12 is a schematic diagram for explaining the dot arrangement
pattern referenced by the index expansion processing unit 67-1 and
67-2 and data conversion methods. The left side of the figure
illustrates a pixel having a 3-value level that is input to the
index expansion processing unit 67-1 or 67-2, and has one of levels
0 to 2. On the other hand, the right side of the figure illustrates
a dot arrangement pattern that is selected to correspond to the
respective level. Each pixel corresponds to two 600 dpi.times.600
dpi sub pixels, and one dot can be printed per one sub pixel.
As illustrated in the figure, when the input level is 0, a dot
pattern is selected in which no dots are printed in either of the
two sub pixels. In other words, 600 dpi.times.300 dpi input data
having level 0 is converted to two 600 dpi.times.600 dpi output
data (level 0 and level 0). When the input level is 1, a dot
arrangement pattern is selected in which a dot is printed in one of
the two sub pixels and no dot is printed in the other. In other
words, 600 dpi.times.300 dpi input data having level 1 is converted
to two 600 dpi.times.600 dpi output data (level 1 and level 0, or
level 0 and level 1). Furthermore, when the input level is 2, a dot
arrangement pattern is selected in which a dot is printed in both
sub pixels. In other words, 600 dpi.times.300 dpi input data having
level 2 is converted to two 600 dpi.times.600 dpi output data
(level 1 and level 1).
Here, focusing in on one 600 dpi.times.600 dpi pixel (on sub
pixel), no black dots are printed in the sub pixel wherein the
binary data 68-1 for the first scan and the binary data 68-2 for
the second scan are both level 0. Moreover, only one black dot is
printed in the sub pixel wherein one of the binary data 68-1 for
the first scan and the binary data 68-2 for the second scan is
level 0 and the other is level 1. Furthermore, two black dots are
overlapped and printed in the sub pixel wherein binary data 68-1
for the first scan and the binary data 68-2 for the second scan are
both level 1.
After that, the binary data 68-1 for the first scan is stored in
the buffer 69-1 for the first scan, and printed in the first scan
by the nozzle array 44 (black nozzle array) in a unit area. Also,
the binary data 68-2 for the second scan is stored in the buffer
69-2 for the second scan, and printed in the second scan by the
nozzle array 44 (black nozzle array) in the unit area. The image
that is printed has a resolution of 600 dpi.times.600 dpi.
The image processing explained in FIG. 1 is explained in more
detail using FIG. 18. FIG. 18 is an image of a detailed example of
the image processing illustrated in FIG. 1. Here, for a resolution
of 600 dpi.times.600 dpi, the case of processing image data 140
that corresponds to 4 pixels.times.4 pixels for a total of 16
pixels is correlated with FIG. 1 and explained. Reference codes A
to P represent combinations of RGB values of image data 140 that
corresponds to each of the 600 dpi.times.600 dpi pixels. In
addition, reference codes ae1 to 1p1 represent combinations of CMYK
values of multi-value data 141 for the first scan that corresponds
to each of the 600 dpi.times.300 dpi pixels, and reference codes
ae2 to 1p2 represent combinations of CMYK values of multi-value
data 142 for the second scan that corresponds to each of the 600
dpi.times.300 dpi pixels.
In FIG. 18, the multi-value data 140 corresponds to the multi-value
RGB data that is input to the color conversion/image data division
unit 62. The multi-value data 141 corresponds to the multi-value
data 64-1 for the first scan as illustrated in FIG. 1, and
multi-value data 142 corresponds to the multi-value data 64-2 for
the second scan. Moreover, the data 143 after quantization
(conversion to three values) for the first scan corresponds to the
quantized data 66-1 for the first scan, and the data 144 after
quantization (conversion to three values) for the second scan
corresponds to the quantized data 66-2 for the second scan.
Furthermore, the binary data 145 corresponds to the binary data
68-1 for the first scan, and the binary data 146 corresponds to the
binary data 68-2 for the second scan.
The following explanation will be in order of processing. First,
the color conversion/image division unit 62 converts the 600
dpi.times.600 dpi input image data 140 (RGB) to multi-value data
141 (CMYK) for the first scan and multi-value data 142 (CMYK) for
the second scan, each having a resolution designated by the
processing resolution setting unit 60. In this embodiment, the
processing resolution setting unit 60 designates a resolution of
600 dpi.times.300 dpi, so the multi-value data 141 for the first
scan and the multi-value data 142 for the second scan have a
resolution of 600 dpi.times.300 dpi.
When doing this, the color conversion/image data division unit 62
calculates the average value of the input image data (RGB) among a
plurality of pixels corresponding to the resolution designated by
the processing resolution setting unit 60. Here, when the
designated resolution is 600 dpi.times.300 dpi as in this
embodiment, the color conversion/image data division unit 62
calculates the average value of input image data (RGB) among two
adjacent pixels in the vertical direction (sub scanning direction)
in the pixels arranged 600 dpi.times.600 dpi. In other words, in
the case of the image data 190 at the upper left in FIG. 18, the
average value Ave (A, E) of A and E is calculated for each of the
three values RGB. After that, by referencing a 3-dimensional LUT
that is prepared in advance, the color conversion/image data
division unit 62 converts the data to four values (CMYK) that
correspond to these three values (RGB), or in other words, converts
the data to multi-value data (ae1) for the first scan and
multi-value data (ae2) for the second scan.
Moreover, as a different method, a 3-dimensional LUT is referenced
for a 600 dpi.times.600 dpi resolution as is, and (CMYK) having a
600 dpi.times.600 dpi resolution can be converted first to two
divisions. In this case, for the two groups of obtained 600
dpi.times.600 dpi CMYK values, the average value is calculated
among a plurality of pixels corresponding to the designated
resolution and taken to be the multi-value data for the first scan
(ae1) and the multi-value data for the second scan (ae2). In other
words, the averaging process based on the resolution designated by
the processing resolution setting unit 60 can be performed before
the color conversion process of converting from RGB to CMYK, or can
be performed after the color conversion process.
The following processing is performed independently and
simultaneously for each color CMYK, so for convenience, the
following explanation will be for only one color (for example,
black (K)), and an explanation of the other colors will be
omitted.
The multi-value data (141) for the first scan is input to the
quantization processing unit 65-1 where it undergoes multi-value
error-diffusion processing and is converted to quantized data (143)
for the first scan. Moreover, the multi-value data (142) for the
second scan is input to the quantization processing unit 65-2 where
it undergoes multi-value error-diffusion processing and is
converted to quantized data (144) for the second scan. When doing
this, error-diffusion processing is performed by the quantization
processing unit 65-1 using the error-diffusion matrix A illustrated
in FIG. 7A, and error-diffusion processing is performed by the
quantization processing unit 65-2 using the error-diffusion matrix
B illustrated in FIG. 7B. In the case of this embodiment, the
quantized data 143 and 144 that is obtained in this way has
resolution of 600 dpi.times.300 dpi, and each pixel is expressed by
three values, 0 to 2.
After that, the quantized data 143 undergoes first scan index
expansion processing and is converted to binary data (145) for the
first scan. Quantized data 144 also undergoes second scan index
expansion processing and is converted to binary data (146) for the
second scan. More specifically, by referencing the dot arrangement
pattern explained using FIG. 12, 600 dpi.times.300 dpi 3-value data
that is expressed by 0 to 2 is converted to 600 dpi.times.600 dpi
binary data.
For examples of binary data 145 and 146, the data of each pixel
that is "1" is data for which a dot is printed (ink is discharged),
and data that is "0" is data for which a dot is not printed (no ink
is discharged). In other words, in binary data 145, for a sub pixel
that is "1", a dot is printed in the first printing scan, and in
binary data 146, for a sub pixel that is "1", a dot is printed in
the second printing scan. By doing so, for a sub pixel that is "1"
and surrounded by a circle, a dot is printed in the first printing
scan and second printing scan such that the dots overlap, and for a
sub pixel that is "1" and is not surrounded by a circle, a dot is
printed in either the first printing scan or second printing scan,
and dots are not overlapped. Therefore, it is possible to have
overlapping dots and non-overlapping dots suitably coexist in the
image, so it is possible to suppress density fluctuation even when
there is deviation in the printing position between the first scan
and second scan.
As explained above, with this embodiment, by dividing multi-value
image data into data for a first scan and second scan and
performing quantization processing, non-overlapping dots and
overlapping dots are suitably mixed, and it becomes possible to
output an image for which density fluctuation and density
unevenness has been suppressed. In addition, when doing this, by
making the resolution before quantization processing temporarily
lower than the input multi-value data, the number of pixels that
become the object of the quantization processing (or in other
words, the number of times quantization is performed) can be kept
from being increased more than necessary. That is, with this
embodiment, in a series of image processing, it is possible to
output a high quality image for which density fluctuation has been
suppressed and with a decrease in processing speed. Furthermore, by
taking the quantization process above to be multi-value
quantization processing, and used together with dot arrangement
pattern processing, the printing resolution can be equivalently
returned to the input resolution, with no drop in the
resolution.
An example of 2-pass multi-pass printing has been explained above,
however, it is also possible of course for this embodiment to
correspond to an even larger number of passes. In this case, the
color conversion/image data division unit 62 divides the image data
that has been converted to low resolution into M number of planes
that correspond to M number of multipasses, and quantization
processing is executed for each of the M number of planes.
However, the method of performing the quantization process after
dividing the data into a plurality of planes as in Japanese Patent
Laid-Open No. 2000-103088 and the present invention is particularly
effective in reducing density unevenness in multipass printing
having a small number of passes (low-pass printing) such as two
passes as described above. That is because the lower the number of
passes, the fewer number of planes there are in which printing is
performed over the same area, and deviation of the printing
position of one plane has a large effect on the image density. On
the other hand, in multipass printing with a large number of
passes, there is a large number of planes in which printing is
performed over the same area, so even though deviation of the
printing position occurs in one or two planes, there is not so much
of an effect on the image density, and density unevenness due to
deviation of the printing position is not as much of a problem as
in low-pass printing.
Moreover, in low-pass printing, high-speed image output is often
desired, so that this embodiment, in which the processing load of
quantization is reduced, is able to have an effect on both the
reduction of density unevenness and an improvement of processing
speed in low-pass printing. On the other hand, in many
pass-printing in which high-quality image output is often desired,
there is a possibility that image quality will decrease due to the
process described above for lowering the resolution. In the process
for lowering the resolution, the original detailed image
information of the pixels is lost due to unification of the
multi-value data of a plurality of adjacent pixels, so after that,
even when returning to the original resolution in the index
expansion process, the lost information does not completely
return.
Therefore, in consideration of the characteristics of the image
processing of this embodiment described above, a plurality of
printing modes having different number of multipasses are prepared
for the printer of this embodiment, and using a different image
processing method depending on the set printing mode is effective.
For example, when the printer is set to the high-speed mode (first
mode), after the process for lowering the resolution and the
division process for dividing the data into M planes, quantization
processing is performed for each plane to generate first quantized
data. After that, an image is printed by M number (M is an integer
2 or greater) of relative movements (M-pass printing) according to
the M number of first quantized data. On the other hand, when the
printer is set to the high-quality mode (second mode), the input
image data is binarized as is using the conventional method and the
and second quantized data are generated without performing the
resolution reduction process and division process. This second
quantized data is divided into N number of data using a mask
pattern that is used in a conventional method, and an image is
printed by N number (N>M) of relative movements (N-pass
printing). By doing so it is possible to obtain both high-speed
output with no density unevenness by using low-pass printing
(M-pass printing), and output a high-quality image by using
many-pass printing (N-pass printing).
In addition, when the printer is set to the high-quality mode
(second mode), it is possible to divide the input image data into M
planes without processing for lowering the resolution. That is,
after dividing the input image data into M planes without
processing for lowering the resolution the error diffusion
processing is executed for each of the M number of planes to
generate binary data. By doing this, an effects of the density
fluctuation can be reduced in the high-quality mode (second
mode).
Embodiment 2
In the following, a second embodiment of the present invention will
be explained. Generally, in a highlighted portion where graininess
is noticeable, it is preferred to evenly disperse the few number of
dots (1701, 1702) such that they have a fixed distance between them
as illustrated in FIG. 8B. Moreover, in high-density portion where
the highest density values are important, it is preferred that dots
be filled in to an extent that there are no exposed areas of blank
paper. In other words, it is desired that the ratio of areas of
overlapping dots (dot overlap rate) be adjusted with good balance
within a range such that density unevenness, graininess and
insufficient density of the output image are not a problem.
However, in the method disclosed in Japanese Patent Laid-Open No.
2000-103088, quantization processing is performed for each plane
without any correlation among the plurality of planes, and the dot
overlap rate is set such that it depends on the quantization
processing method used. In other words, in the method disclosed in
Japanese Patent Laid-Open No. 2000-103088, it was possible to set
the dot overlap rate such that it was not 0, however; it was not
possible to adjust the dot overlap rate within a desired range.
For example, in the construction disclosed in Japanese Patent
Laid-Open No. 2000-103088, locations (1603) where dots overlap, and
adjacent printed locations (1601, 1602) occur here and there as
illustrated in FIG. 8C, so clumps of these dots stand out and cause
graininess to become worse. In addition, in high-density areas
where maximum density values are important, blank areas may become
exposed due to overlapping of dots, and this may cause insufficient
density. As a result, it is difficult to output an image at high
speed in which there is no problem with density unevenness,
graininess and insufficient density in any of the density
portions.
In order that this embodiment can handle new problems such as this
in addition to the problems that the invention is to solve as
explained above, an image processor that is constructed in order to
more actively adjust the dot overlap rate of dots that are printed
by different planes (printing scanning) is explained.
Here, the term "dot overlap rate` as used in this specification
will be explained. "Dot overlap", as illustrated in FIGS. 4A to 4H
and FIG. 10, is the ratio of the number of dots (overlapping dots)
that are to be printed in the same location by different printing
scans or different printing element groups to the total number of
dots to be printed per unit area of K (K is an integer 1 or
greater) number of pixels. The same location in the case of FIGS.
9A to 4G is the same pixel location, and in the case of FIG. 10 is
the sub pixel location.
Using FIGS. 4A to 4H, the dot overlap rate for a first plane and
second plane that correspond to a unit area comprising 4 pixels
(main scanning direction).times.3 pixels (sub scanning direction)
is explained. The "first plane" is a collection of binary data that
correspond to the first scan or first nozzle group, and the "second
plane" is a collection of binary data that correspond to the second
scan or second nozzle group. Moreover, "1" represents data that
indicates that a dot will be printed, and "0" represents data that
indicates that a dot will not be printed.
In FIGS. 4A to 4E, the number of "1s" in the first plane is "4" and
the number of "1s" in the second plane is also "4", so the total
number of dots to be printed in a unit area comprising 4
pixels.times.3 pixels is "8". On the other hand, the number of "1s"
in the first plane and second plane that correspond to the same
pixel location is the number of dots (overlapping dots) that is
printed in the same pixel such that they overlap. According to this
definition, the number of overlapping dots in the case of FIG. 4A
is "0", in the case of FIG. 4B is "2", in the case of FIG. 4C is
"4", in the case of FIG. 4D is "6" and in the case of FIG. 4E is
"8". Therefore, as illustrated in FIG. 4H, the dot overlap rates in
FIGS. 4A to 4E are, 0%, 25%, 50%, 75% and 100%, respectively.
Furthermore, FIGS. 4F and 4G illustrate cases in which the number
of printed dots in a plane and number of total dots differ from the
cases of FIGS. 4A to 4E. FIG. 4F illustrates the case in which the
number of dots printed in the first plane is "4", the number of
dots printed in the second plane is "3", the total number of dots
is "7", the number of overlapping dots is "6" and the dot overlap
rate is 86%. On the other hand, FIG. 4G illustrates the case in
which the number of dots printed in the first plane is "4", the
number of dots printed in the second plane is "2", the total number
of dots is "6", the number of overlapping dots is "2 and the dot
overlap rate 33%.
In this way, the term "dot overlap rate" in this specification is
the overlap rate of dot data when dot data corresponding to
different scans or different printing element groups overlap
virtually.
The image processing method of the present invention for
controlling the dot overlap rate is explained in detail. In this
embodiment, as illustrated in FIG. 11B, a printing head 5004 is
used that comprises two nozzle arrays (print element groups) for
each color. In the figure, 51 and 58 are cyan nozzle arrays, 52 and
57 are magenta nozzle arrays, 53 and 56 are yellow nozzle arrays
and 54 and 55 are black nozzle arrays. By arranging the nozzle
arrays for each color such that they are symmetric in the main
scanning direction as in this example, it is possible to fix the
order of ink application to the print medium of the printing scan
in the forward direction and of the printing scan in the backward
direction. In this embodiment, 2-pass multipass printing is
executed using two nozzle arrays for one color such as this.
FIG. 14 is a block diagram for explaining the steps of the image
processing that is executed in this embodiment. In this embodiment
as well, RGB multi-value image data that is input from an external
device to the multi-value image data input unit 71 is such that
individual pixels for each color is expressed by 8-bit (256 values)
and has a 600 dpi.times.600 dpi resolution. In addition, the color
conversion/image data division unit 72 converts this input image
data (256-value RGB data) to two sets of multi-value image data
(256-value density data) that correspond to the ink colors (CMYK).
However, in the color conversion/image data division unit 72, with
bias being applied according to a specified distribution rate, the
image data is divided into image data for the first scan and image
data for the second scan. A feature of this embodiment is
controlling the dot overlap rate by adjusting the distribution rate
of the color conversion/image data division unit 72 in this way.
The relationship between the distribution rate and dot overlap rate
will be explained in detail later.
In this embodiment as well, the processing resolution setting unit
71 sets the resolution of the image data after division by the
color conversion/image data division unit 72. For example, when the
processing resolution setting unit 70 designates a resolution of
600 dpi.times.300 dpi, the color conversion/image data division
unit 62 output two groups of 256-value density data that
corresponds to the ink colors (CMYK) according to a specified
distribution rate and with a resolution of 600 dpi.times.300 dpi.
As a result of performing this kind of process, it is possible to
complete the gradation correction process and quantization process
by performing processing the same number of times as the
conventional method in which the division process is not performed
(the method disclosed in Japanese Patent Laid-Open No. 2000-103088
is not employed) the same as in the first embodiment.
The processing described below is performed independently and
simultaneously for each of the colors CMYK, so the following
explanation will be for just the color black (K). In the color
conversion/image data division unit 72, multi-value data for the
first scan and multi-value data for the second scan that are
generated according to a specified distribution rate undergo a
gradation correction process as in the first embodiment by the
gradation correction units 73-1 and 73-2, respectively. After that,
as in the first embodiment, the quantization processing units (75-1
and 75-2) perform quantization processing and send the data to the
printer engine 3004 via an IEEE 1284 bus 3022. The printer engine
3004, then executes the following processing.
In the printer engine 3004, 3-value data 76-1 for the first scan,
and 3-value data 76-2 for the second scan undergo index expansion
processing as in the first embodiment explained in FIG. 12. By this
processing, the 3-value data 76-1 and 76-2 are converted to 600
dpi.times.600 dpi binary data 78-1 and 78-2 that is expressed by
two values of 1-bit.
After that, the binary data 78-1 is input to the first scan binary
data division processing unit 79-1 and divided into binary data
80-1 for the first nozzle array in the first scan, and binary data
80-2 for the second nozzle array in the first scan. Moreover, the
binary data 78-2 is input to the second scan binary data division
processing unit 79-2 and divided into binary data 80-3 for the
first nozzle array in the second scan, and binary data 80-4 for the
second nozzle array in the second scan. In this embodiment, in the
first scan binary data division processing unit 79-1 and second
scan binary data division processing unit 79-2, the division
processing is executed using masks that are stored in memory in
advance. A mask is a collection of data which set permitting to
print (1) or not permitting to print (0) for each binary image data
of the 600 dpi.times.600 dpi pixels, and by performing a logical
AND operation between the mask and the binary image data for each
pixel the binary data is divided.
In the case where the binary image data are divide into N
divisions, normally N number of masks are used, and in this
embodiment where the binary image data are divided into two
divisions, two masks 1801 and 1802 as illustrated in FIG. 5 are
used. Here, mask 1801 is used for generating binary data for the
first nozzle array, and mask 1802 is used for generating binary
data for the second nozzle array. These two masks have a
complementary relationship with each other, so the binary data that
are divided using these masks will not overlap each other.
Therefore, compared with the two planes that were divided out by
the color conversion/image data division unit 72, a plane
relationship occurs in which it is more difficult for graininess to
become worse and for insufficient density to occur. In FIG. 5, the
black areas are data (1: data for which the image data is not
masked) for which printing image data is permitted, and white areas
are data (0: data for which the image data is masked) for which
printing image data is not permitted. In this embodiment, the same
set of masks 1801 and 1802 are used by the first scan binary data
division processing unit 79-1 and second scan binary data division
processing unit 79-2. However, as long as the masks for the first
nozzle array and second nozzle array have a complementary
relationship with each other, it is also possible to use a
different set of masks for the first scan and second scan.
After that, the binary data (80-1) for the first nozzle array in
the first scan is stored in a buffer (81-1) for the first nozzle
array in the first scan, and printed in a unit area in the first
scan by nozzle array 54. The binary data (80-2) for the second
nozzle array in the first scan is stored in a buffer (81-2) for the
second nozzle array in the first scan, and printed in a unit area
in the first scan by nozzle array 55. The binary data (80-3) for
the first nozzle array in the second scan is stored in a buffer
(81-3) for the first nozzle array in the second scan, and printed
in a unit area in the second scan by nozzle array 54. Furthermore,
the binary data (80-4) for the second nozzle array in the second
scan is stored in a buffer (81-4) for the second nozzle array in
the second scan, and printed in a unit area in the second scan by
nozzle array 55. The printed image has a resolution of 600
dpi.times.600 dpi.
The method and effect of adjusting the distribution rate for
controlling the dot overlap rate is explained below. Table 1 gives
the distribution rates for the multi-value data for the first scan
and second scan by the color conversion/image data division
processing unit 72 of this embodiment, and dot overlap rates of the
first scan and second scan when typical error diffusion processing
is performed on the multi-value data.
The printing rate (%) in Table 1 corresponds to the number of dots
of one color of ink that is printed per unit area, where 0% is the
state in which no dots are printed per unit area, and 100% is the
state in which the maximum number of dots is printed per unit area.
Therefore, for example, a printing rate of 60% indicates a state in
which the number of dots printed per unit area corresponds to a
value (153) that is 60% the maximum value (255) of multi-value data
K that is input to the color conversion/image data division unit
72. In Table 1, this printing rate is given in 10 levels from 0 to
100%.
Moreover, the distribution rate (%) is the ratio of the multi-value
data set according to the printing rate that is distributed to the
first scan (K1) and the second scan (K2), where the total
distribution rate is 100%. For example, when distributing the
multi-value data value (K=100) to multi-vale data (K1=80) for the
first scan, and multi-value data (K2=20) for the second scan, the
distribution rate is (K1:K2=80:20). In Table 1, this kind of
distribution rate is given in 6 levels. The dot overlap rates,
which are the result of quantization processing by the typical
error-diffusion method and the index expansion process illustrated
in FIG. 12, are given in each column corresponding to the
distribution rate and printing rate.
TABLE-US-00001 TABLE 1 Distribution rate (%) Printing rate (%)
First scan Second scan 10 20 30 40 50 60 70 80 90 100 100 0 0 0 0 0
0 0 0 0 0 0 90 10 1.8 3.6 5.4 7.2 9 10.8 12.6 14.4 16.2 18 80 20
3.2 6.4 9.6 12.8 16 19.2 2.4 25.6 28.8 32 70 30 4.2 8.4 12.6 16.8
21 25.2 29.4 33.6 37.8 42 60 40 4.8 9.6 14.4 19.2 24 28.8 33.6 38.4
43.2 48 50 50 5 10 15 20 25 30 35 40 45 50
FIG. 6 is a diagram that illustrates Table 1 as a graph. The
horizontal axis is the printing rate, the vertical axis is the dot
overlap rate, and for each of the 6 levels of distribution rates
illustrated in Table 1, the dot overlap rate with respect to the
printing rate is expressed as a straight line with a different
slope. For example, when the distribution rate for the first
printing scan is 100% and the distribution rate for the second
printing scan is 0%, all of the multi-value data is printed by just
the first printing scan. In this case, there is no dot overlap, and
even though the printing rate increases the dot overlap rate
remains at 0%. As the distribution rate for the second printing
scan gradually increases, the slope of the dot overlap rate with
respect to the printing rate gradually increases. In addition, when
both the distribution rate for the first printing scan and second
printing scan is 50%, the slope of the dot overlap rate with
respect to the printing rate becomes a maximum, and when the
printing rate is 100%, the dot overlap rate becomes 50%. Therefore,
by obtaining the relationship between the distribution rate and dot
overlap rate in advance as illustrated in Table 1 and FIG. 6, it is
possible to achieve the desired dot overlap rate by adjusting the
distribution rate.
In FIG. 6, the thick line 311 illustrates the state in this
embodiment of adjusting the dot overlap rate according to the
printing rate (or in other words the total value of a plurality of
multi-value density data corresponding to different scans). In this
embodiment, in low-density portions where the printing rate is up
to 20%, the dot overlap rate is 0%, in medium density portions
where the printing rate is 20 to 60%, the dot overlap rate
gradually increases to 30%, and in high-density portions where the
printing rate is 60% or greater, the dot overlap rate gradually
drops to 20% or less. In order to achieve this kind of dot overlap
rate, the distribution rate at a printing rate of 0 to 20% is
(100%:0%), and at a printing rate of 20 to 60%, the distribution
rate is gradually changed until it becomes (50%:50%). In addition,
at a printing rate of 60%, in order that the dot overlap rate
becomes a maximum, the distribution rate is made to be (50%:50%).
Moreover, in high-density areas where the printing rate is 60 to
100%, the distribution rate is gradually changed unit it becomes
(90%:10%). In this embodiment, the distribution rate is set in this
way such that the dot overlap rate in medium-density portions,
where density unevenness due to density fluctuation caused by
deviation of the printing position is considered to be the most
important, is higher than in low-density portions and high-density
portions. In order to suppress the occurrence of pseudo contours,
it is preferred that adjustment of the distribution rate be changed
as smoothly as possible with respect to change in the printing
rate.
Incidentally, in this embodiment, the color conversion/image data
division unit 62 converts all of the input image data (RGB)
together to a plurality of multi-value density data corresponding
to the printing scans, so parameters such as those that correspond
to the "printing rates" illustrated in Table 1 and FIG. 6 are not
actually used. However, there is a correlation between the total
value of multi-value data for the first printing scan and
multi-value data for the second printing scan after distribution
and the printing rate, and when this total value becomes large, the
printing rate after binarization becomes large. In other words, the
total value of a plurality of multi-value density data that
corresponds to different scans corresponds to the "printing rate".
Therefore, in actual processing, RGB coordinates can be correlated
with CMYK values in a 3-dimensional LUT so that the multi-value
data at the coordinates where the total value of multi-value data
for each printing scan corresponds to a printing rate of 60% is
distributed for each printing scans at the distribution rate of
(50:50). In other words, in this embodiment, the color
conversion/image data division unit 72 performs data conversion by
using a LUT table in which the RGB coordinates are correlated with
CMYK multi-value data so that the relationship between the total
value of multi-value data for each printing scan (printing rate)
and the distribution rate satisfies the graph indicated by the
thick line in FIG. 6. By doing so, it is possible to set the dot
overlap rate for medium-density portions, where density unevenness
is considered to be the greatest, higher than low-density portions
and high-density portions, and for all density portions where the
priority between density unevenness and graininess changes, it is
possible to output a good image.
Also, in this embodiment, as in the first embodiment, the color
conversion/image data division unit 72 does not have to convert all
of the multi-value brightness data (RGB) to a plurality of
multi-value density data (CMYK) at once. The process for converting
color from RGB to CMYK can be provided separately from the process
of converting CMYK data to a plurality of multi-value density data
that corresponds to printing. In that case, a plurality of
multi-value density data can be generated according to the
distribution rate that is set according to the multi-value data
after color conversion, or in other words, according to Table
1.
The process for lowering the resolution, as in the first
embodiment, can be performed in any stage as long as it is
performed before the quantization process. In other words, the
color conversion process for converting RGB to CMYK can be
performed in the RGB stage after calculating the average value of
multi-value data among a plurality of adjacent pixels, and the
process for lowering the resolution can be performed after the
color conversion process.
In Table 1, the color conversion/image data division unit 72 sets
the distribution rates such that the sum of the distributions rates
for the first printing scan and second printing scan becomes 100%,
however; this embodiment is not limited to this. With the
convenience of the image processing or the object of improving the
absolute density, the sum of the distribution rate for the first
printing scan and the distribution rate for the second printing
scan can be greater than 100%, or may be less than 100%.
With the embodiment explained above, by performing quantization
processing after dividing multi-value density data for a first
printing scan and second printing scan according to a suitable
distribution rate, it is possible to set the dot overlap rate in
density portions where density unevenness is considered to be more
of a problem higher than in other portions. When doing this, by
making the resolution after division lower than that of the input
multi-value density data, it is possible to keep from increasing
the number of times quantization processing is performed more than
necessary. Therefore, in a series of image processing, it is
possible to output an image for which various problems, including
density unevenness, are suppressed for all of density portions
without lowering the processing speed.
Embodiment 3
In the second embodiment, a method was explained in which the color
conversion/image data division unit adjusts the distribution rate
in order to control the dot overlap rate. In this embodiment, the
dot overlap rate is controlled by providing features to the
quantization process when quantizing the plurality of multi-value
density data that is generated by the color conversion/image data
division unit. When doing this, the dot overlap rate may also be
controlled by the quantization processing unit cooperating with
adjustment of the distribution rate by the color conversion/image
data division unit.
FIG. 15 is a block diagram for explaining image processing when
performing multipass printing for completing an image in the same
area of a printing medium by two printing scans. In this
embodiment, except for the quantization processing unit 25, the
construction is the same as that of the second embodiment that was
explained using FIG. 14. Therefore, only the processing executed by
the quantization processing unit 25 will be explained here.
FIG. 9 is a flowchart for explaining processing steps that are
executed by the quantization processing unit 25 of this embodiment.
In this flowchart, the two objects to be quantized, or in other
words, input multi-value data 24-1 for the first scan and input
multi-value data 24-2 for the second scan are indicated as K1 and
K2, respectively, and have values 0 to 255. In addition, K1err and
K2err are accumulated error values that are generated from
surrounding pixels for which quantization processing has already
been completed, and K1ttl and K2ttl are totaled values of the input
multi-value data (K1, K2) and accumulated error values (K1err,
K2err). Furthermore, K1' and K2' in the flowchart indicate 3-value
quantized data for the first printing scan and second printing
scan.
In this embodiment, threshold values (quantization parameters) that
are used when setting the 3-value quantized data K1' and K2',
differ depending on the values K1ttl and K2ttl. For this purpose,
the values K1ttl and K2ttl are taken to be arguments, and a
threshold value table is prepared in advance from which two
threshold values, which have different sizes, are selected
corresponding to these arguments. Here, when setting K1',
K1table_1[K2ttl] is a first threshold value for comparison with
K1ttl, and K1table_2[K2ttl] is a second threshold value that is
larger than the first threshold value. Moreover, when setting K2',
K2table_1[K1ttl] is a first threshold value for comparison with
K2ttl, and K2table_2[K1ttl] is a second threshold value that is
larger than the first threshold value. Both K1table_1[K2ttl] and
K1table_2[K2ttl] are values that are selected from the threshold
value table with K2ttl as an argument. Both K2table_1[K1ttl] and
K2table_2[K1ttl] are values that are selected from the threshold
value table with K1ttl as an argument.
When this process is started, first, in step S21 K1ttl and K2ttl
are calculated. Next, in step S22, by referencing the threshold
value table, the four threshold values are obtained from K1ttl and
K2ttl that were calculated in step S21. The threshold values
K1table_1[K2ttl] and K1table.sub.--2[K2ttl] are primarily set by
referencing the table with K2ttl as an argument. Also, the
threshold values K1table_2[K1ttl] and K2table_2[K1ttl] are
primarily set by referencing the table with K1ttl as an
argument.
Next, in steps S23 to S26, the value K1' is set, and in steps S27
to S30, the value K2' is set. More specifically, in step S23, the
value K1ttl that was calculated in step S21 is compared with the
threshold values K1table_1[K2ttl] and K1table_2[K2ttl] that were
obtained in step S22. When it is determined that
K1ttl<K1table_1[K2ttl], then K1'=0, and the accumulated error
value K1err (=K1ttl) is updated according to this output value
(K1'=0) (step S24). Moreover, when it is determined that
K1table_1[K2ttl].ltoreq.K1ttl.ltoreq.K1table_2[K2ttl], then K1'=1,
and the accumulated error value K1err (=K1ttl-128) is calculated
according to this output value (K1'=1) and updated (step S25).
Furthermore, when it is determined that K1table_2 [K2ttl]<K1ttl,
then K1'=2, and the accumulated error value K1err (=K1ttl-255) is
calculated according to this output value (K1'=2) and updated (step
S26).
Next, in step S27, the value K1tt2 that was calculated in step S21
is compared with the threshold values K2table_1[K1ttl] and
K2table_2[K1ttl] that were obtained in step S22. When it is
determined that K2ttl<K2table.sub.1[K1ttl], then K2'=0, and the
accumulated error value K2err (=K2ttl) is updated according to this
output value (K2'=0) (step S28). Moreover, when it is determined
that K2table_1[K1ttl].ltoreq.K2ttl.ltoreq.K2table_2 [K1ttl], then
K2'=1, and the accumulated error value K2err (=K2ttl-128) is
calculated according to this output value (K2'=1) and updated (step
S25). Furthermore, when it is determined that
K2table.sub.--2[K1ttl]<K2ttl, then K2'=2, and the accumulated
error value K2err (=K2ttl-255) is calculated and updated according
to this output value (K2'=2) (step S30).
After that, in step S31, the accumulated error values K1err and
K2err that were updated in this way are diffused to the surrounding
pixels for which quantization has not yet been completed according
to the error-diffusion matrix illustrated in FIG. 7A or FIG. 7B. In
this embodiment, the error-diffusion matrix illustrated in FIG. 7A
is used for diffusing the accumulated error K1err to the
surrounding pixels, and the error-diffusion matrix illustrated in
FIG. 7B is used for diffusing the accumulated error K2err to the
surrounding pixels.
In this embodiment, the two threshold values (quantization
parameters) that are used for performing the quantization process
on the first multi-value data (K1ttl) that corresponds to the first
scan are set based on the second multi-value data (K1ttl) that
corresponds to the second scan. Similarly, the two threshold values
(quantization parameters) that are used for performing the
quantization process on the second multi-value data (K2ttl) that
corresponds to the second scan are set based on the first
multi-value data (K1ttl) that corresponds to the first scan. In
other words, the quantization process of multi-value data that
corresponds to one scan of two scans, and the quantization process
of multi-value data that corresponds to the other scan of two scans
are both executed based on the multi-value data that corresponds to
the one scan and the multi-value data that corresponds to the other
scan. In this way, for example, the table contents can be adjusted
in advance so that dots that are printed in one scan are as much as
possible not printed in the pixels in which dots are printed in the
other scan, or vice versa. As a result, it is possible to increase
the suppression of density unevenness according to gradation, while
at the same time keep a balance between worsening graininess and
insufficient density.
FIG. 13 is a diagram for explaining the correlation between an
example of the results from performing quantization processing
(binarization processing) using threshold values that are given in
a specified threshold value table according to the flowchart in
FIG. 9 and the input values (K1ttl and K2ttl). Both K1ttl and K2ttl
take on values 0 to 255, and depending on the combinations of
these, K1' and K2' are set as illustrated in the figure to 2-dot
printing (2), 1-dot printing (1) or no printing (0). Here, the
first threshold value K2table_1[K1ttl] that is used for
quantization of K2ttl is indicated by the bold dotted line, and the
second threshold value K2table_2[K1ttl] is indicated by the bold
dashed line. For example, for a pixel in which both K1' and K2'
become 2, that is, for a pixel corresponding to the upper right
area of the figure, two dots are printed in 1.times.2 sub pixels in
the first printing scan and second printing scan respectively. When
K1' is 1 and K2' is 2, or in other words, in a pixel that
corresponds to the area on the right side of the figure, 1 dot is
printed in a 1.times.2 sub pixel in the first printing scan, and
two dots are printed in a 1.times.2 sub pixel in the second
printing scan. When both K1' and K2' are 0, or in other words, in a
pixel that corresponds to the area at lower left of the figure, no
dots are printed in either the first scan or second scan.
In this embodiment, a table is prepared in this way such that the
threshold values change according to the values K1ttl and K2ttl, or
in other words, a table is prepared for setting two threshold
values for comparison with K2ttl (or K1ttl), using K1ttl (or K2ttl)
as an argument. By doing this, as illustrated in FIG. 13, it is
possible to change the threshold values for comparison with K2ttl
(or K1ttl) according K1ttl (or K2ttl), and as a result, it is
possible to control the dot overlap rate in 1.times.2 sub
pixels.
It is also possible to prepare a plurality of kinds of threshold
value tables in advance in order to obtain various results in
addition to the results illustrated in FIG. 13, and by doing so, it
is possible to obtain a suitable dot overlap rate according to
various printing conditions such as the type of printing medium. As
a result, it is possible to print good images on various kinds of
printing media, for which problems in the image due to graininess,
insufficient density and density unevenness have been suppressed in
all density portions from low-density areas to high-density
portions.
In the explanation above, the case of preparing a threshold table
from which threshold values for quantization can be selected from
arguments (K1ttl, K2ttl) was explained, however, the quantization
method is not limited to the method described above. Quantization
does not have to be such that output values (0 to 2) are set by
comparison with threshold values. For example, in the case of two
planes, a table can be prepared that primarily sets the values K1'
and K2' by using both K1ttl and K2ttl as reference values. The
details of such a table are omitted here, however, using a
multi-dimensional table such as this has merits in that the control
is more simple, and the dot overlap rate can be controlled with a
higher degree of freedom. On the other hand, using a 1-dimensional
threshold value table such as described above has merits in that it
is possible to create a table that requires less memory space. It
is also possible to use a 1-dimensional threshold value table that
uses the sum of K1ttl and K2ttl as an argument. Furthermore, it is
also possible to perform quantization processing by just performing
branching and calculations without using a table at all. In that
case, it is possible to obtain the effect of this embodiment by
setting the coefficients that are used in the operations to values
that make it possible to achieve the desired dot overlap rate. In
such a case, it is possible to further reduce the amount of
required memory space (size of ROM or RAM used) when compared with
the case of preparing the table described above.
With the embodiment as explained above, quantization processing for
each scan is performed according to multi-value data of each scan
after dividing multi-value density data into data for a first scan
and second scan. By doing so, it is possible to set the dot overlap
rate higher in density portions where density unevenness is
considered to be more of a problem than in other portions. When
doing this, by making the resolution after this division process
lower than that of the input multi-value density data, it is
possible to prevent having to increase more than necessary the
number of times quantization processing is performed. Therefore, in
a series of image processing, it is possible to output an image for
which various problems, including density unevenness, have been
suppressed for all of density portions without a decrease in
processing speed.
Other Embodiments
In the first through third embodiments, the case was explained in
which processing is performed by lowering the resolution (600
dpi.times.300 dpi) in just the sub scanning direction with respect
to the resolution (600 dpi.times.600 dpi) of the image that is
input to the multi-value image data input unit. However, the degree
to which the resolution is lowered can be changed in various ways
according to a setting from the processing resolution setting unit.
For example, the resolution (600 dpi.times.600 dpi) could be
lowered in just the main scanning direction (300 dpi.times.600
dpi), or could be lowered in both the main scanning direction and
sub scanning direction (300 dpi.times.300 dpi).
When the resolution is lowered in just the sub scanning direction
as in the embodiments described above, an index pattern that is
long in the sub scanning direction as illustrated in FIG. 12 is
used. By using this kind of index pattern, it is possible to obtain
an image that is resistant to a deviation in the printing position
in the main scanning direction generally. On the other hand, when
the resolution is lowered in just the main scanning direction such
that the resolution is changed from (600 dpi.times.600 dpi) to (300
dpi.times.600 dpi), a dot arrangement pattern that is long in the
main scanning direction is used as illustrated in FIG. 17. By using
this kind of dot arrangement pattern, it is possible to obtain an
image that is resistant to a deviation in the printing position in
the sub scanning direction generally. The load on the quantization
process itself is nearly the same whether the resolution is lowered
in the vertical or horizontal direction. Therefore, the processing
resolution setting unit can set the processing resolution according
to the direction in which it is desired to further reduce the print
deviation.
Moreover, when the color conversion/image data division unit lowers
the resolution from (600 dpi.times.600 dpi) to (300 dpi.times.300
dpi), for example, the number of pixels that become the object of
quantization processing is kept at 1/4(=1/2.times.1/2) that of when
the resolution is not lowered. In this case, one 300 dpi.times.300
dpi pixel corresponds to four 600 dpi.times.600 dpi sub pixels, so
in the quantization process, quantization to five values 0 to 4 may
be performed.
FIG. 16 is a diagram illustrating one example of a dot arrangement
pattern that is used in the index expansion process. The left side
of the figure illustrates pixels having 5 level values that are
input to the index expansion process, and have one of the levels 0
to 4. On the other hand, the right side of the figure illustrates
dot arrangement patterns for 2.times.2 sub pixels that are selected
to correspond to the respective levels. When the level is 0, no
dots are printed in any of the 2.times.2 sub pixels, however, as
the level increases, then number of dots printed increases by one.
In level 1 to level 3, a plurality of patterns having different dot
arrangements are prepared, and these can be used sequentially or
can be used selectively according to conditions. This kind of
resolution reduction and index expansion processing can be applied
to any of the embodiments 1 to 3.
The number of levels after quantization in the quantization process
does not absolutely need to correspond to the number of sub pixels.
For example, even though the resolution is lowered from (600
dpi.times.600 dpi) to (300 dpi.times.300 dpi), in the quantization
process, quantization can be performed to three values instead of
five values. In that case, the dot arrangement patterns that
correspond to the levels could be for example, 0 dots in level 0, 2
dots in level 1, and 4 dots in level 3.
Control of the dot overlap rate that was made possible in the
second and third embodiments can be made possible by using the
index expansion process as explained in FIG. 10. FIG. 10 is
schematic diagram for explaining the relationship between 3-value
quantized data and the dot arrangement pattern that is converted by
the index expansion processing unit in accordance to the first scan
and second scan. The left side of the figure illustrates pixels
having levels of three values that are input to the index expansion
processing unit 27-1 or 27-2, with each pixel having a value 0 to
2. Each pixel has a resolution of 300 dpi.times.300 dpi, which
corresponds to 2.times.2 sub pixels having a resolution of 600
dpi.times.600 dpi. On the other hand, the center columns in the
figure illustrate dot arrangement patterns of the 600 dpi.times.600
dpi 2.times.2 sub pixels that are selected according to the
respective level. The dot arrangement pattern that corresponds to
K1' is the dot arrangement pattern for the first scan, and the dot
arrangement pattern that corresponds to K2' is the dot arrangement
pattern for the second scan. In the figure, the sub pixels that are
indicated by diagonal lines indicate that one dot is printed in
that sub pixels during the specified scan.
The right side of the figure illustrates the sum of the dot
arrangement pattern for the first scan and the dot arrangement
pattern for the second scan (dot overlap state) that is set in thus
way, and the overlap rate for each respective case. In the figure,
sub pixels for which no dots are printed in either the first scan
or second scan are illustrated as empty, and sub pixels for which
one dot is printed in either the first scan or second scan are
illustrated using diagonal lines. Furthermore, sub pixels for which
one dot is printed in both the first scan and second scan for a
total of two overlapping dots are illustrated by a solid black. In
this way, it is possible to set the dot arrangement for the dot
arrangement pattern in advance so that the desired dot overlap rate
is obtained. In addition, control of the dot overlap rate that uses
this kind of index expansion processing can be performed together
with control of the dot overlap rate by a different method such as
that used in the first embodiment of course, as well at that used
in the second and third embodiments.
When the resolution is lowered from (600 dpi.times.600 dpi) to (300
dpi.times.300 dpi) as described above, the number of pixels that
become the object of the quantization process becomes approximately
1/4 that when the resolution is not lowered. In other words, in
4-pass multipass printing, when divided into four planes, the
number of pixels that are the object of processing can be made to
be nearly the same (4.times.1/4=1) as the case in which division
processing is not performed. In addition, in 2-pass multipass
printing, when divided into two planes, the number of pixels can be
made to be 1/2 (2.times.1/4=1/2) that of the case in which division
processing is not performed, and thus even faster processing can be
expected. Generally, when performing M-pass printing (M is an
integer 2 or greater), by making the processing resolution 1/m
times in the main scanning direction and 1/n times in the sub
scanning direction such that m.times.n=M is satisfied, it is
possible to perform the quantization process with the same load as
when the division process was not performed even though the
division process was performed to divide the data into M planes.
However, the present invention is not limited to lowering the
resolution so that m.times.n=M is satisfied. In image construction
in which the quantization process is performed after dividing the
data into a plurality of planes, as long as the image is
constructed by performing the quantization process after lowering
the resolution to a resolution that is lower than the input
resolution, it is possible to obtain the effect of the present
invention of reducing the processing load that accompanies dividing
the data. In this case, in index expansion after the quantization
process, the resolution can be expanded to a resolution that is
higher than the input resolution (increased resolution).
However, when lowering the resolution, the original detailed image
information of pixels is lost due to making the image data in a
plurality of adjacent pixels uniform, so after that, even though
the original resolution is restored by index expansion processing,
the lost information is not completely restored. Therefore, when
the amount of resolution reduction (values of m and n) is very
large, a decrease in image quality will stand out. In consideration
of this, the resolution set by the processing resolution setting
unit, or in other words, the amount of resolution reduction is
preferably set to a suitable value that balances the number of
division planes, the processing load, the required processing
speed, and the image quality.
In the embodiments described above, the case was explained in which
the color conversion/image data division unit converts the
multi-value brightness data (RGB) to a plurality of multi-value
density data (CMYK) corresponding to the printing scans in one step
by using a 3-dimensional LUT. However, the present invention and
this embodiment are not absolutely limited to this kind of
construction. For example, the process of converting the color from
RGB to CMYK and the process of dividing CMYK to a plurality of
multi-value density data that corresponds to the printing scans can
be independent. In this case, it does not matter whether the
resolution conversion and the division process are performed in the
RGB state before color conversion, or resolution conversion and the
division process are performed in the CMYK state after color
conversion. The resolution conversion process and division process
could also be separately located before and after the color
conversion process, where the before and after relationship of the
resolution conversion process and division process is not
particularly limited. In either case, in the stage of performing
the quantization process, as long as multi-value CMYK data for a
plurality of scans in which the processing resolution is made to be
lower than the input resolution, it is possible to obtain the
effect of the present invention of reducing the load of the
quantization process.
Moreover, in the second and third embodiments, with the presumption
that a printing head having two nozzle arrays of each color as
illustrated in FIG. 11B will be used, the case was explained in
which data was divided in the multi-value stage into a plane for
the first printing scan and a plane for the second printing scan.
In addition, after the data for each scan is converted to binary
data by the index expansion process, that binary data is divided
into data for the first nozzle array and for the second nozzle
array by using a mask pattern having a complementary relationship.
This kind of construction is effective when density unevenness
between printing scans stands out more than density unevenness
between nozzle arrays. However, the present invention is not
limited to this kind of construction. When the density unevenness
between nozzle arrays stands out more than the density unevenness
between printing scans, and when the deviation in the printing
position between nozzle arrays is emphasized more, it is possible
to perform the division process in the opposite order as described
above. In that case, the quantization process and index expansion
process can be performed after performing division in the
multi-value state to a plane for the first nozzle array and a plane
for the second nozzle array, and after that, binary data can be
divided for the first scan and for the second scan using a mask
pattern.
In the embodiments described above, the case was explained in which
four colors of ink CMYK were used, however, the number of types of
ink colors that can be used are not limited to this. Light cyan
(Lc), or light magenta (Lm) ink could be added to these 4 colors of
ink, or special color ink such as red (R) or blue (B) ink could
also be added. On the other hand, the present invention could also
be applied to a mono-color mode in which only one color of ink is
used. Furthermore, the invention could also be applied to a
monochrome printer as well as a color printer.
Moreover, in the embodiments described above, the case of using a
printer having the construction as illustrated in the electrical
block diagram of FIG. 3 was explained, however; the present
invention is not limited to this kind of construction. For example,
the printer control unit and printer engine unit were explained as
each being an independent module, however, the control unit and
printer engine unit can share the same ASIC, CPU, ROM and RAM. In
addition, in the figure, the control unit and printer engine unit
are connected by a USB or IEEE 1284, which is a general-purpose
I/F, however, the present invention could use any connection
method. Moreover, the connection from the PC takes the form of a
direct connection to the printer engine unit via a USB HUB,
however, the control unit could also relay the image data.
Furthermore, as necessary, the control unit could employ the form
of sending image data from the PC to the printer engine unit after
performing suitable image processing on the image data.
In the embodiment above, the case was explained in which image
processing up to quantization is executed by the control unit 3000,
and processing after that is executed by the printer engine 3004,
however, the present invention is not limited to this kind of
construction. As long as the series of processes described above
are executed, any form of processing method, regardless of the
hardware or software, is within the scope of the present
invention.
Furthermore, in the embodiments described above, using a printer
(image formation device) comprising a control unit 3000 having an
image processing function as an example, an image process device
that executes image processing that is characteristic of the
present invention was explained, however the present invention is
not limited to this kind of construction. A construction is
possible in which the characteristic image processing of the
present invention is executed by a host device in which a printer
driver is installed (for example PC 3010 in FIG. 3) and image data
after quantization processing or division processing is input to
the printer. In such a case, the host (external device) that is
connected to the printer corresponds to the image processor of the
present invention.
The present invention can also be achieved by program code that
comprises a program readable by a computer for making possible the
image processing functions described above, or a memory medium on
which that program code is stored. In this case, the computer (or
CPU or MPU) of the host device or the image processor make possible
the image processing described above by reading and executing the
aforementioned program code. In order to make the computer execute
the image processing described above in this way, the present
invention includes a program that can be read by the computer, or a
memory medium on which that program is stored.
It is possible to use, for example, a floppy disk (registered
trademark), hard disk, optical disk, magnetic optical disk, CD-ROM,
CD-R, magnetic tape, non-volatile memory card, ROM and the like as
the memory medium for supplying the program code.
Moreover, by the computer reading and executing the program code,
not only is it possible to achieve the functions of the embodiments
described above, but based on the instruction of that program code,
the OS operating on the computer can also perform part or all of
the actual processing. Furthermore, after the program code has been
written in the memory of a function expansion board that is
inserted into the computer, or a function expansion unit that is
connected to the computer, the CPU can perform part or all of the
processing based on the instructions of that program.
Aspects of the present invention can also be realized by a computer
of a system or apparatus (or devices such as a CPU or MPU) that
reads out and executes a program recorded on a memory device to
perform the functions of the above-described embodiment(s), and by
a method, the steps of which are performed by a computer of a
system or apparatus by, for example, reading out and executing a
program recorded on a memory device to perform the functions of the
above-described embodiment(s). For this purpose, the program is
provided to the computer for example via a network or from a
recording medium of various types serving as the memory device
(e.g., computer-readable medium).
While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. The scope of
the following claims is to be accorded the broadest interpretation
so as to encompass all such modifications and equivalent structures
and functions.
This application claims the benefit of Japanese Patent Application
No. 2009-235328, filed on Oct. 9, 2009, which is hereby
incorporated by reference herein in its entirety.
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